fiber gas kiln design question

Hi, I'm looking for help with my kiln design. I am having trouble getting it to temperature most of the time. It is ceramic fiber, cylindrical, approximately 50 c.f. and propane fueled with a 500 gal tank. I don't have the exact btu available, but I believe it is more than enough, with a single burner. I primarily fire it with a saggar, which is made of hard brick, also cylindical, that sits and fits exactly on a 26" round shelf. The fire box is about 4 inches tall, made of soft brick, which supports the fiber shell. The gap between the bottom shelf and the wall varies between 3 and 4 inches while the gap between the saggar wall and shell is about 3.5 inches. The gap between the top of the saggar and the roof can vary from between 3 and 7 inches, with little difference noted in performance. It generally peaks out at about 1800 deg., however when it windy, I can get it up to 2200+ without any trouble, so obviously there is an air flow issue. I have experimented with innumerable combinations of vent and intake size, including a flue, although most of the time I haven't had a flue. It seems once I get the vent large enough, I just start losing heat. Is my only solution a blower? My other thought is to rebuild the roof so that it is more of an arch. Any help is very much appreciated. Thanks.

For fiber kiln questions email Lenny at [email protected] or go to www.ceramicservices.com or phone 909 986 1566 (California). Also Cynthia Schiff knows about firing a fiber kiln as I almost bought hers from her in 2012. Cynthia can be reached by calling 818 903 4366 (California, Los Angeles area). Hope you get your problem solved.

I need some more info
You say you you have no flue? How and where does the heat get out?
Kilns need to breath-
Also 50 cubic feet is huge for one burner-It must be a massive one.
How is the heat/flame getting into the sagger?
Is this an updraft or downdraft kiln or no draft?
Mark

In addition to Mark's questions, how thick is the fiber? As Mark says, 50 cubic ft. is a good size kiln for one burner. Can you post a picture? a schematic drawing of the air flow? What type of burner? Size of flu?

If the saggar sits on a 26" shelf, and the gap between the saggar and the shell is 3.5 inches, then then the floor of this kiln is about 6 square feet. So the kiln is not 50 cubic feet unless is it 8 1/2 feet tall. You'll need to calculate the actual cubic footage of the kiln, and know the actual btu's of the burner before we can really make any educated guesses as to what's going on here. If you are using a venturi burner, then you may need to create some sort of flue/chimney to create more draft and get some secondary air in there.

The burner is the MR100 venturi I purchased from Ward Burner Systems after consulting with them. The fiber is 2" of 6lb. and 1" of 8lb.

It is an updraft kiln, with a flue. I was using the term flue when I should have said chimney. So, I tried a chimney at one point, but generally don't have one. I believe the specs I found suggested a port size of 1 1/2 times the size of the burner and the flue I started with was approximately 4-5 inches by 4-5 inches, but I ended up making it slightly larger and then adjusting the size during firing by draping pieces of fiber over the edges to cut down the size. When firing, I start with the burner inside the firebox during the candling and gradually move the burner to the outside of the kiln to about an inch away. Like I said, I've experimented a lot with the flue and port sizes, but with little effect. I've tried adding a separate port just for extra air as well. Also, the kiln is in two parts, so I can fire using just the top half, which is probably about 10 c.f., which also doesn't seem to affect the overall temp capabilities.

What you've got there is basically a raku kiln, which is not really designed to go to cone 10. You can get just about any pile of bricks or fiber up to raku temps. So you'll probably have to make some adjustments to get this to function as a high fire kiln.

Another inch of fiber wouldn't hurt.

I assume the burner is coming in from the side? I would make the firebox twice as tall for starters. It may be choked off down there. How thick are the fire box walls? They should be two layers of brick thick, 9 inches, with staggered joints so it seals well. A leaky firebox will lose too much heat and screw up the air flow. And put a small target brick right in the middle of the firebox to disperse the flame.

The size of the flue opening will not affect the draft that much, it will only affect the pressure in the kiln. To create the necessary draft you need to have a vertical chimney. This could be made out of wire mesh and fiber, or just a metal tube of some sort. With a fiber shell I imagine you won't be able to set much weight up there, though. The chimney will create a draft that will draw in air through the burner port.

My cone 10 car kiln door is all fiber and its over 6 inches thick-3 inches is a raku kiln-The single burner burner and no chimney is what most raku kilns are.
You need to redesign this for a cone 10 kiln if thats what you want from this-more fiber a chimney for draw or not if its a true updraft and maybe another burner..
In a cone 10 kiln you do not move the burner in or out as its fixed in raku you can do do all that.
Check out a few kiln books for some ideas. Cone 10 needs more fiber-3 inches is not enough.
Neil's ideas will get you on the right track.
Mark

Thanks to everyone for your help on this. I'm not actually looking to get to cone 10, I was just illustrating that I have enough in the BTU dept. to get to there on a windy day. I'd be happy with cone 06, probably. 04 hopefully, just a hard bisque. But, I will add some fiber, bulk up and increase the fire box size, and try a chimney again. Thanks again!

If you're just going to 04, you can probably get away with the firebox being one brick (4.5") thick. You just have to make sure the bricks are set tight together at 90 degree angles. You have to have the full thickness of the brick at each corner. No setting them at angles to make a circle. However making it taller wouldn't hurt. There's no such thing as too tall a firebox in this case.